You're only looking at one very narrow aspect of organic growing.
Organic growing, and animal husbandry, take the view that prevention is much better than cure. So a good grower will concentrate on disease prevention through plant health, good waste management, proper rotations, hygiene practises, weed management, wildlife management and so on. Derris, bordeaux mixture and other inputs should only be used as a last resort, not a some sort of "approved-therefore-harmless" prophylactic that can be sprayed around willy-nilly.
A good inspector will not only look at what is being grown, but also how the farm is being managed, and will mark a grower down for poor weed control etc. If you were regularly spraying potatoes with bordeaux mixture when it wasn't necessary, your organic status could be suspended.